Determine whether the following function is one-to-one on its domain. If it is one-to-one, find its inverse function $f^{-1}(x)$. If it is not-one-to-one, write "undefined" in the blank provided. You do not need to simplify your answer.

  $f(x) = x^2 - 6$

  $f^{-1}(x) =$